This invention relates to improvements in electrical power assisted steering systems, and in particular to an improved apparatus for measuring the absolute steering angle of the road wheels.
Typical electric power assisted steering systems comprise a steering shaft operatively connected at a first end to a steering wheel and at its opposite end operatively connected to the roadwheels of a vehicle. An electric motor is provided which can apply torque to the steering shaft through a reduction gearbox. The gearbox may be of the worm and wheel, or other, type.
A steering gear is provided between the steering shaft and the steered wheels. This steering gear typically provides a substantial gearing between angular movement of the steering shaft (and hence hand wheel) and the movement of the roadwheels. For a typical road vehicle, more than one turn a of the handwheel (i.e. complete revolution of the steering shaft) is needed to move the roadwheels from lock to lock.
It is desirable to be able to measure the steering angle of the roadwheels. This can be used to influence a number of sub-systems in the vehicle such as suspension damper control systems, vehicle stability control systems and vehicle lane guidance.
One solution to the problem would be to provide an angular position sensor on the steering shaft to measure the angle of rotation of the steering shaft. However, as more than one fill revolution is needed to turn from lock to lock such a measurement would not unambiguously describe the angle of the roadwheels.
The problem of measuring multiple turns of the steering shaft can be overcome in several ways, each with its own disadvantage.
In one proposal, the steering shaft angular position sensor can be driven by the steering shaft through a step down gear, reducing the total number of turns of the sensor to less than one full revolution. This overcomes the problem of ambiguity, but unfortunately reduces the resolution which can be obtained from the sensor. To produce a high resolution system is therefore expensive.
According to the present invention, we provide an electric power assisted steering system comprising: a steering shaft operatively connected at a first end to a handwheel and at its other end operatively connected to at least one roadwheel, an electric motor having a rotor operatively connected to the steering shaft through a gearbox having a non-integer reduction gear ratio, a first sensing means adapted to produce an output dependent on the angular position of the steering shaft; a second sensing means adapted to produce an output dependent on the angular position of the rotor, and processing means adapted to process both output signals to produce an angular position signal indicative of the angular position of the steering shaft over a range of greater than one complete revolution.
The invention thus employs outputs from two sensors, one monitoring the position of the steering shaft and the other monitoring the position of the motor rotor to provide, if desired, an unambiguous measurement of steering shaft angle over a range of angles in excess of one full revolution.
Preferably, both sensors are adapted to produce a cyclic output signal dependent upon angular position which repeats after a complete revolution, or perhaps a fraction of a full revolution. The cycle may repeat upon a complete rotation of the associated steering shaft or motor rotor, i.e. 1 cycle corresponds to 360xc2x0 of rotation. For example, one sensor may produce an absolute angular position value which varies substantially linearly over the range 0-360 degrees of rotation between a value of 0 and 1. The sensor will therefore produce the same output value for shaft or rotor positions of 90xc2x0, 90xc2x0+360xc2x0, 90xc2x0+720xc2x0 etc. Alternatively, it may have a range of 0-180xc2x0, and thus the cycle will repeat itself once within a single revolution.
At least one of the sensors may comprise an absolute angular position sensor. By this we mean that the sensor produces a signal that represents the absolute angular position of the shaft or rotor within a complete revolution (or part of a revolution). Examples of sensors of this kind include potentiometers, a resolver, a synchro and an optical angle encoder. For clarity, it is assumed that the absolute sensor produces an output that varies substantially linearly between 0 and 1 over its range of output values.
Alternatively, at least one of the sensors may comprise an index sensor. By this we mean a sensor which is adapted to produce an output signal dependent on angle which is indicative of the position of the shaft within a small fraction of a revolution. Such a sensor may, for example produce a short pulse as the shaft rotates past its index position, and zero output in all other positions. Again, more than one index pulse may be produced within a single revolution, i.e. two equal-spaced pulses per complete revolution.
Preferably, the sensors are driven directly from the steering shaft or motor rotor without intermediate gearing. Thus, with a sensor having a cycle of 360 degrees, for one turn of the shaft the sensor measures one full revolution.
Preferably, the gear ratio may be expressed as p/q whereby the motor turns through p/q revolutions for each revolution of the steering shaft, p is greater than q, q is greater than unity, and the greatest common integer factor of p and q is also unity.
By gearbox ratio, we mean the ratio between the rotation of the two sensors. Thus, if each sensor produces an output value over a range corresponding to one full revolution, the gear ratio is the turns ratio between the input side and output side of the gearbox. If one sensor produces an output which cycles or repeats n times within one revolution of its respective shaft or rotor, the gearbox ratio will be npxe2x80x2/q where npxe2x80x2=p as herein before.
By selecting a non-integer ratio, the outputs of the two sensors will drift out of synchronisation as the steering shaft rotates. Eventually, after a predetermined number of revolutions, the output will return into synchronisation This xe2x80x9cbeatingxe2x80x9d enables an unambiguous measurement of rotation over a range greater than one revolution to be achieved from sensors which produce an output over a range of one revolution or less.
In one arrangement, the first sensing means comprises an absolute handwheel position sensor and the second sensing means comprises an index sensor adapted to produce an index signal at a known angular position of the motor rotor, said processing means being adapted to sample the output of the first sensing means corresponding to the position when the second sensing means produces an index signal;
multiply the sampled value by p;
round the multiplied value to the nearest integer to produce a reference value and
use the reference value to access the corresponding entry in a look-up table, said entry being indicative of the number of revolutions of the steering shaft from an arbitrary zero position.
In another arrangement, the first sensing means may comprise an index sensor adapted to produce an index signal at a known angular position of the handwheel with the second sensing means comprising an absolute position sensor.
In yet a further alternative, both sensing means may comprise absolute position sensors. Again, the processing means is adapted to exploit the way in which the outputs of the sensors drift out of synchronisation and back into synchronisation after a number of revolutions to obtain a measure of the number of rotations of the steering shaft from an arbitrary zero angular position. A benefit of using two absolute position sensors is that it is no longer necessary to wait until one of the sensors passes an index, allowing a more regular estimate of position to be made.
In the event that both sensors comprise absolute position sensors, the processing means may be adapted to estimate the angular position of the motor rotor from a measurement of the angular position of the steering shaft assuming it is on its xe2x80x9czeroxe2x80x9d revolution. This estimate may then be compared with the actual output signal from the second sensing means, and the difference between the estimate and actual values processed to produce a signal indicative of the number of revolutions of the steering, shaft relative to an arbitrary zero angular position.
The processing means may therefore, in one system, be adapted to multiply the measured steering shaft position value output from the first sensing means by the gear ratio p/q to produce a predicted motor shaft position, sample the actual motor position from the second sensing means, compare the predicted value to the actual measured steering position, calculate the difference between the measured value and predicted value, and process the difference value to produce a value indicative of the number of turns of the steering shaft.
The processing means may be further adapted to calculate a residue of the difference and multiply the residue by q. This multiplied value may then be rounded off to the nearest integer, and the rounded value used to access a look-up table.
It is envisaged that the apparatus can be modified in a number of ways. For example, the second sensing means may in one arrangement comprise a number of Hall effect sensors adapted to detect the angular position of one or more magnets on the motor rotor.
In a most preferred arrangement, the motor may comprise a brushless permanent magnet motor and the motor sensor may comprise a number of Hall effect sensors adapted to detect the position of the magnetic poles. Three sensors may be provided for a three phase motor. This allows a resolution of ⅙th  of an electrical revolution of the rotor. For instance, with a 3 phase motor with 6 poles, the output will repeat 3 times for one whole revolution of the motor rotor.
The second sensing means may also be used to provide position information for use by a motor control circuit. For example, it can be used to calculate the timing for motor commutation events.
In accordance with a second aspect, the invention provides an electric power assisted steering system comprising a steering shaft operatively connected to one or more roadwheels and an electric motor adapted to apply an assistance torque to the shaft which incorporates a means adapted to check the relationship between the actual angular position of the steering angle and the expected angular position of the road wheel carriers.
The straight ahead position will vary in service. Specifically, the relationship between the angle or linear position of the steering system components may change due to wear or deformation of the chassis components, adjustment of the steering or suspension components or the replacement of steering system components. By checking the relationship between actual and measured angle such changes can be detected and compensated or corrected as necessary.
The measured angular position of the steering shaft may be produced using an electric power assisted steering system which embodies the first aspect of the invention.
It is envisaged that there are several preferred ways of achieving the checks by recognising that the vehicle is travelling in a straight line which are described below. Any number of these can be combined to detect if the vehicle is travelling in a straight line. If the methods detect that the absolute steering angle does not correspond with the straight-ahead detection then the offset on the absolute steering angle signal can be changed or the angle detection means can be stopped and a fault indicated.
1. The system may further include a yaw sensor adapted to detect that the vehicle is travelling in a straight line. The system may be adapted to measure the output of a first sensing means which comprises an absolute steering angle position sensor. It may then calculate an offset to correct the absolute steering angle signal so that it indicates the straighdt-ahead condition when the vehicle is travelling in a straight line.
The yaw sensor may be adapted to indicate a straight line travel when the quantity:
|indicated yaw|/indicated vehicle speed
is below a certain threshold for a period greater than a certain time. The, threshold and the duration can be chosen for the vehicle to which the system is applied. xe2x80x9c|.|xe2x80x9d indicates absolute value. The calculation must be protected from the case when the vehicle speed is zero, for example calculation may be disabled at low vehicle is speeds. The calculation may perhaps only be used when the rate of change in the vehicle speed is low.
2. The system may be adapted to decide that the vehicle is travelling in a straight line by monitoring the values of handwheel velocity and handwheel torque. An electric power steering system may therefore further include means for monitoring the handwheel velocity and means for monitoring the handwheel torque. The system may then be adapted to determine if the absolute value of the handwheel velocity is below a threshold, the absolute value of handwheel torque is below a threshold and the vehicle speed is above a threshold. In this condition it is highly likely that the steering system will be pointing substantially straight ahead. This condition can be made more discriminating by screening out the cases when the vehicle speed, handwheel torque or handwheel velocity are changing at a high rate.
3. Use a steering angle that is averaged over distance. The system therefore includes means for monitoring the average direction of travel of the vehicle. This is very close to straight ahead when large distances are considered. Therefore accumulating an average of the steering angle over distance will show if the absolute steering angle is well-aligned with the true straight-ahead. A low-pass filter may be provided that is adapted to filter the output of a steering shaft angular position sensor with respect to distance. This can be approximated by a time-based filter but a time-based filter will not work correctly at low vehicle speeds. A better approach is to let the input to the filter be an angle xcex1, the filter output be an average angle A, the filter xe2x80x9ctime-constantxe2x80x9d be k, the distance travelled be x and the vehicle velocity v. Then a first order low-pass filter that operates over distance is:                     A        =                  ∫                                    k              ⁡                              (                                  α                  -                  A                                )                                      ⁢                          ⅆ              x                                                              =                  ∫                                    k              ⁡                              (                                  α                  -                  A                                )                                      ⁢                                          ⅆ                x                                            ⅆ                t                                      ⁢                          ⅆ              t                                                                        =                      ∫                                          k                ⁡                                  (                                      α                    -                    A                                    )                                            ⁢              v              ⁢                              ⅆ                t                                                    ⁢                  xe2x80x83                    
The input angle may be compared with the filter output to generate an error signal. The error is multiplied by the xe2x80x9ctime-constantxe2x80x9d and the vehicle speed and is then integrated (over time). Thus, when the vehicle speed is zero the filter output will not change. When the vehicle speed is high the filter output will adapt quickly. This filter can be incorporated into the absolute steering angle detection scheme by applying the filter to, the absolute steering angle output; the output of the filter (after an appropriate settling time) can be used to detect the offset that the absolute steering angle has from zero. The offset may be stored in non-volatile memory and restored into the filter integrator for use on the next journey that the vehicle makes.
The stored offset should be bounded to prevent an excessive value being used. If the filter output exceeds a pre-determined limit, then it may be desirable to disable the absolute steering angle detection scheme until it has been inspected at a service point.
Other Epas Drive Systems
There are other suitable cases in which two sensing means, each adapted to produce an output of angular position are geared with respect to one another by a train of gears and one sensor rotates with the handwheel and the other sensor rotates at a higher rate. The system of the first aspect may be modified to suit each case. The other cases to consider are listed below:
Pinion-drive: the first sensing means may be located on the driver side of a pinion shaft of a rack-and-pinion steering gear. The motor may then be adapted to drive the pinon instead of the steering shaft via a reduction gearbox as described hereinafter. Of course, this falls within the meaning of the term xe2x80x9coperatively connectedxe2x80x9d to the steering shaft, as will be apparent to the skilled person.
Rack-drive: the first sensing means may be located on the driver side of a pinion shaft of a rack-and-pinion steering gear. The motor drives the rack directly through some gear-train that converts the motor""s rotary motion to a linear motion (typically, this is a recirculating ball-nut that drives a lead-screw machined into the rack). The second sensing means may thus be geared to the rack which is geared to the pinion. The gear ratio between the motor and the pinion will be:
pinion revolutions per mm/motor revolutions per mm
Dual-pinion drive: this is a special case of the rack-drive in which the motor is adapted to drive the rack through a second pinion. The handwheel is connected to the first pinion and the first sensing means is mounted on the input shaft of the first pinion. The motor drives the second pinion via a reduction gearbox. Thus the gear ratio between the motor and the xe2x80x9ccolumnxe2x80x9d angle sensors is:
motor reduction ratioxc3x97second pinion ratio/first pinion ratio
Although this is a more complex chain, provided it has a non-integer ratio, the rack position detection method can be employed.
The electric power assisted steering system of the first aspect of the invention produces an angular position signal for the steering shaft. There are many envisaged uses for this absolute steering angle signal. Protection may be sought for any of these uses. These include:
1. Providing a xe2x80x9cpowered-centeringxe2x80x9d function in which the electrical motor is adapted to provide a torque to the steering shaft which returns the steering system to the straight-ahead position. The system may be adapted to produce a torque demand that is added to the normal assistance torque demand to provide xe2x80x9cpowered-centeringxe2x80x9d that acts to return the road wheels to the straight ahead position when the driver releases the handwheel. For example when the steering system is rotated to steer the vehicle left, a torque that acts to turn the steering to the right may be added to the normal assistance torque and vice versa when the steering is turned to the right.
2. Enabling xe2x80x9csoftxe2x80x9d steering end-stops in which the EPAS system is adapted to drive the motor with an assistance torque which is reduced when the steering system is near to the end-stops. This prevents the driver from rotating the steering system quickly onto the end-stop and so xe2x80x9csoftxe2x80x9d end-stops can reduce the shock loads and the associated noise with hitting the end-stop of the steering. Clearly this can be combined with the powered-centering function. The torque may be added in the same manner as the powered-centering torque.
3. Providing a signal for use by a xe2x80x9cVehicle Dynamic Controlxe2x80x9d system that aims to control the yaw of a vehicle by braking different wheels. A VDC system computes the yaw that is required by the driver from the absolute steering angle and the vehicle speed; the actual yaw of the vehicle is measured by a yaw sensor and the difference between the measured and demanded values is used to control the distribution of the brake force to correct the yaw error. The absolute steering angle can be used as an input to the VDC controller.
4. Providing a signal for use by a damping control system in which the suspension damper units are xe2x80x9cstiffenedxe2x80x9d when the vehicle is cornering. The absolute steering angle signal can be used to give advanced warning that the driver is entering a corner and the damping rate can be increased before the vehicle starts to roll. Once the vehicle is travelling in a straight course, the damping rate may be reduced to give a soft ride.
5. Providing a signal for use by a steering angle control system. Such a system may use a closed-loop feedback controller to generate an EPAS assistance torque that depends on the difference between a demanded steering angle and the absolute steering angle. The demanded steering angle may arise from some vehicle guidance system, for example, this could be a signal from a camera that determines the course of the road by recognising the lane markings or a signal from some roadside equipment that indicates the direction of the road.
It is envisaged that in at least one arrangement the invention may be successfully implemented in combination with an alternate scheme for detecting the steering angle. The system will be adapted to base its measurements on the output of one or other of the systems depending upon prevailing conditions such as recovery from battery failure.
One particular alternative system which it is envisaged could be used alongside the present invention is described in our earlier British application No. GB 9900774.7 filed on the Jan. 15, 1999. The disclosure of the earlier application is fully incorporated herein by reference, and is referred to as the xe2x80x9cmotor position counterxe2x80x9d system whilst for clarity the system described herein relating to the first aspect of the present invention is referred to as xe2x80x9cnon-integer gear sensorxe2x80x9d system.
The earlier dated application discloses an electrical power steering system in which the output from a motor position sensor, typically comprising a number of Hall effect devices, is combined with an index signal from a sensor connected to the steering shaft or the rack or perhaps a yaw sensor to produce an accurate measurement of steering angle by counting transitions in the output of the Hall effect sensors. The index sensor produces an index signal and the counter is reset when the index is produced to ensure the count does not drift out.
Thus, in accordance with a further aspect the invention provides an electric power-assisted steering system according to the first aspect of the invention in which the second sensing means is further adapted to produce an output signal indicative of angular position of the rotor which undergoes periodic transitions as the rotor rotates, the processing means being adapted to produce a second angular position signal indicative of the angular position of the steering shaft by counting transitions in the output of the second sensing means, the count being reset whenever the output of the first sensing means corresponds to an index position of the steering shaft, the processing means being adapted to combine both the first and second angular position signals to produce an authoritative angular position signal.
The invention of this aspect thus combines all of the features of the first aspect (producing a first position signal) with those of the invention described in G139900774.7 and provides an authoritative output based on the output of one or other of the systems.
The processing means may combine the first and second angular position signals by normally using the second angular position signal to produce the authoritative output whilst using the first angular position signal to verify the second position signal.
If the two angular position signals differ the output produced by the first aspect of the invention may be used as the basis of the authoritative output. This may continue to be used until the first sensing means produces an index signal and the count is reset. At this time it is known that the count is correct. This allows the system to produce an authoritative output after a fault when the count would otherwise be incorrect for the entire period of operation until the steering shaft rotates such that an index is produces (i.e. the output of the first sensor on the steering shaft is at the index position).
In an alternative the first angular position signal may be used to reset the count signal without waiting for the steering shaft to cross the index position. This can be done if the first angular position signal is deemed to be reliable. This situation may arise upon power-up, when the count may have been lost yet the first aspect of the invention produces an immediate reliable output.
It will, of course, be appreciated that the combination of both systems can provide a more accurate and reliable system whilst also providing valuable cross checking. As the systems share physical sensors hardware is minimised. Of course, one physical sensor may produce more than one output, i.e. an incremental output and a continuous output.
Both systems can be implemented so that they share physical sensors. In use the output of one system may have advantages over that of the other. The following method has been proposed:
On power-up, when the power steering system is first energised, the output determined by the motor position counter system is read. This initialises the authoritative angular position signal used within the control system. The output of the column position sensor, which measures absolute position, is used to incrementally update this signal as it has a higher resolution. The output produced by the non-integer gear sensor system is then used as a cross-check to detect steering wheel turn. If there is a large difference between the two signals, then the non-integer gear sensor system output can be used to reset the motor position counter system output and/or the system can stop using the position signal for the remainder of the journey.
In another situation, if a battery fault occurs and the motor position counter system output is lost (i.e. is unreliable) the counter signal will be invalid and so cannot be used at power-up. In this case the steering angle is not available until the non-integer gear sensor system can identify the correct steering wheel turn. As soon as the turn is identified the system can resort to using the motor position counter system by resetting the counter when the steering is in the straight ahead position. The normal operation described in the preceding paragraphs, based on the count system, can then be resumed, the non-integer gearing method being used as a back-up for cross-checking.